1. Field of the Invention
This invention relates to a zinc phosphate-treated galvanized steel sheet, having excellent corrosion resistance and color tone, that can be used for applications such as household electric appliances, building materials, automobiles and so forth.
2. Description of the Related Art
Zinc phosphate-treated steel sheets for use in automobiles, household electric appliances and building materials have been subjected in the past to zinc phosphate treatment, chromate treatment and organic coating treatment to improve qualities such as corrosion resistance and coating performance. However, the use of the chromate-treated steel sheets, in particular, has been prevented in recent years, to prevent possible environmental pollution, because it may contain hexavalent chromium, and the demands on zinc phosphate treatments have increased.
When the zinc phosphate treatment is used alone, however, resistance to white rust and resistance to non-uniform dew condensation are entirely insufficient, and a rinsing treatment in a hexavalent chromium-containing aqueous solution called xe2x80x9csealing chromate treatmentxe2x80x9d, xe2x80x9csealing treatmentxe2x80x9d or xe2x80x9cpost-treatmentxe2x80x9d, has been generally carried out. From the aspect of environmental preservation, therefore, the zinc phosphate treatment is essentially not sufficient, and the development of a method capable of improving the white rust resistance and the non-uniform dew condensation resistance, and being free of any environmental problem, has been desired.
A uniform white appearance is required in most cases for the zinc phosphate-treated galvanized steel sheets. If the color tone of an underlying film is dark when white coating is applied, the color tone after coating becomes dark, too, and the commercial value drops.
To cope with this problem, Japanese Patent No. 2,770,860, for example, describes a treatment method using a formation treating solution containing Zn ions and Mg ions in specific concentrations. Since this method is based on the premise of the sealing chromate treatment, however, the corrosion resistance is not sufficient. Japanese Unexamined Patent Publication (Kokai) No. 9-49086 describes a phosphate treatment method providing a steel sheet having high whiteness and excellent in adhesion by using a formation treatment solution containing Ni ions and Mg ions in specific concentrations. According to this method, however, the corrosion resistance, particularly white rust resistance, and the non-uniform dew condensation resistance are not sufficient and the whiteness and its uniformity are-not sufficient, either. Japanese Unexamined Patent Publications (Kokai) No. 1-312081 and No. 3-107469 disclose a zinc plated steel sheet having an Mg-containing phosphate film. The corrosion resistance, particularly white rust resistance, and the non-uniform dew condensation resistance are not sufficient in this method. Though this method provides high whiteness, this method is not sufficient from the aspect of uniformity of the color tone. WO00/73535 describes a zinc plated steel sheet that has a phosphate film containing Mg and Ni or Mn formed thereon and is excellent in the corrosion resistance (red rust resistance) and coating performance. When directed to obtain uniform white appearance and excellent white rust resistance and non-uniform dew condensation resistance, however, this method is not entirely satisfactory.
This invention aims at providing a zinc phosphate-treated galvanized steel sheet that solves the problem described above, does not require the sealing chromate treatment, can secure sufficient white rust resistance and non-uniform dew condensation resistance, has high whiteness and is excellent in color tone.
The present inventors have examined the influence of metal components other than Zn in the zinc phosphate-treated film formed on the galvanized steel sheet, and have found that the white rust resistance, non-uniform dew condensation resistance, whiteness and color tone uniformity can be improved when a relatively large amount of Mg is contained, but this means is not sufficient when it is alone used. When suitable amounts of Ni, Co and Cu, that are electrochemically more noble than Zn, are allowed to precipitate beside Mg, these elements operate as the nuclei with the result that a compact and uniform film can be formed, and the object of the present invention can be achieved.
The gist of the present invention resides in a zinc plated steel sheet having a zinc phosphate film, wherein the zinc phosphate film contains at least 2% of Mg and 0.01 to 1% of at least one element selected from the group consisting of Ni, Co and Cu, and the adhesion value of the film is at least 0.7 g/m2.
The galvanized steel sheet used in the present invention is not particularly limited, and both pure zinc plating and alloy plating can be used. The plating means is not particularly limited, either, and electroplating, hot-dip plating, vacuum deposition, and so forth, can be employed.
In the zinc phosphate film formed on the galvanized plating, Mg and an element selected from the group consisting of Ni, Co and Cu must essentially co-exist as metal components other than zinc. The amount of Mg is at least 2% and the amount of at least one kind of elements selected from the group consisting of Ni, Co and Cu must be from 0.01 to 1%. When the amount of either one of these components is below the respective lower limits, the white rust resistance and the non-uniform dew condensation resistance are deteriorated and uniformity of the color tone drops. When co-present, these elements form a compact and uniform white skin film having less pits. The upper limit of at least one kind of elements selected from the group consisting of Ni, Co and Cu is set to 1% because the color tone becomes dark and uniformity of the color tone drops when the amount exceeds this upper limit of 1%.
The adhesion value of the zinc phosphate film is at least 0.7 g/m2. When the amount is less than this value, the white rust resistance and the non-uniform dew condensation resistance drop.
The zinc phosphate type galvanized steel sheet according to the present invention does not require a sealing chromate treatment after the zinc phosphate treatment. A sealing treatment not containing Cr is naturally advantageous from the aspect of corrosion resistance, but is not necessary.
The zinc phosphate film of the invention can be formed by conducting an ordinary treatment such as spraying or dipping in an ordinary zinc phosphate treating bath containing phosphoric acid ions, Zn ions or nitric acid ions and fluorides by adding ions of Mg, Ni, Co, Cu, etc, in the form of nitrates to the treating bath.